Touch sensors detect touch, such as from a user's finger, a stylus, or some other object. Touch sensors may be transparent or opaque input devices for computers or other electronic systems. A transparent touch sensor, such as a touchscreen, may be used in conjunction with a display device. Touch displays are increasingly used in commercial applications, such as restaurant order entry systems, industrial process control applications, interactive museum exhibits, public information kiosks, pagers, cellular phones, computers, personal digital assistants, and video games.
Acoustic-based sensors may be used to detect touch. Certain types of acoustic touchscreens, also known as ultrasonic touchscreens, detect touch with high transparency and high resolution, while providing a durable touch surface. Of particular commercial interest are ultrasonic touchscreens using surface acoustic waves. FIGS. 16A and 16B show an example of a conventional wedge transducer 26 for generating surface acoustic waves that all propagate in a direction represented by arrow 5. A piezoelectric element 30 mounted on the top back surface (also referred to as a first surface) of the wedge generates bulk pressure acoustic waves, represented by rays, in the wedge 28. At the boundary surface of the wedge 28 that is on or in contact with the touch substrate 10, surface acoustic waves, represented by ray 5, are generated that propagate across the touch substrate 10.
A surface acoustic wave touchscreen includes a substrate on which the surface acoustic waves propagate. X and Y transducers generate surface acoustic waves along perpendicular axes. Transmit reflective arrays proximate the edges of the touch substrate and spaced along the axes reflect the transmitted surface acoustic waves across the touchscreen's touch surface along perpendicular paths. Receive reflective arrays proximate the edges of the touch substrate and spaced along opposite sides from the transmit reflective arrays reflect the surface acoustic waves that have propagated across the touch surface to X and Y receive transducers or sensors. These reflective arrays may be referred to as “linear reflective arrays”, and acoustic waves traveling in a linear direction partially pass through the linear reflective array and partially are reflected by the linear reflective array in a direction normal to the linear direction. When a touch occurs on the touch surface, the touch causes attenuation of the surface acoustic waves at corresponding locations along the two axes, X and Y. The X, Y touch position is determined based on the timing of the attenuation in the signal received at the receive sensors.
With some conventional surface acoustic wave touch sensors, multiple simultaneous touches may be difficult to correctly locate due to ambiguity. The multiple touches cause the detection of two X and two Y attenuation coordinate locations, so that it may be unclear which detected X location is associated with a particular detected X, Y location. Associating the correct combination of X and Y locations together to determine the proper coordinates requires a guess or more information.